General Characteristics

Two well-described Sarcocystis species include S. bovihominis (cattle) and S. suihominis (pigs). (Some publications refer to S. bovihominis as S. hominis.) When uncooked meat from these infected animals is ingested by humans, gamogony (fission resulting in the production of sporozoan gametes) can occur in the intestinal cells, with eventual production of the sporocysts in stool.

Sarcocystis spp. have an obligatory two-host life cycle. Intermediate hosts (herbivores and omnivores) become infected through ingestion of sporocysts excreted in the feces of the definitive hosts (carnivores and omnivores). The definitive hosts become infected through ingestion of mature cysts found in the muscles of the intermediate hosts. In some intermediate hosts, such as cattle and sheep, all adult animals may be infected. Extraintestinal human sarcocystosis is rare, with a much lower incidence than is seen with the intestinal infection. Humans who have ingested meat containing the mature sarcocysts serve as the definitive hosts. Fever, severe diarrhea, abdominal pain, and weight loss have been reported in immunocompromised hosts, although the number of patients with these symptoms has been quite small.

The sporocysts found in the stool are broadly oval and slightly tapered at the ends. They measure 9 to 16 µm long and contain four mature sporozoites and the residual body (see Table 1). Normally, the oocyst contains two sporocysts (similar to I. belli); however, in Sarcocystis infections, the sporocysts are released from the oocyst and normally are seen singly. These sporocysts tend to be larger than Cryptosporidium oocysts that contain four sporozoites, so they look totally different. The oocysts are fully sporulated when passed in the stool.

Table1. Morphologic Criteria Used to Identify Intestinal Protozoa (Coccidia, Blastocystis hominis)

Pathogenesis and Spectrum of Disease

 When humans (intermediate host) ingest oocysts from other animal stool sources, the sarcocysts that develop in human muscle are 7 to 16 µm long and cause few, if any, problems. Basically, no inflammatory response to these organisms occurs in the muscle, and no evidence of pathogenicity is seen. Patients demonstrate symptoms related to the disintegration of the sarcocysts and death of intracystic bradyzoites. Painful muscle swellings measuring 1 to 3 cm in diameter are associated with erythema of the overlying skin; these occur periodically and last 2 days to 2 weeks. Symptoms also include fever, diffuse myalgia, muscle tenderness, weakness, eosinophilia, and bronchospasm. Different types of skeletal and cardiac muscle sarcocysts have been found in humans. No specific therapy is required for this type of infection. Corticosteroids can reduce allergic inflammatory reactions.

Infections in humans can manifest primarily as intestinal disease if infected meat is ingested or as muscular disease if sporocysts are ingested. Intestinal disease occurs within a few hours after consumption of infected meat and is characterized by nausea, abdominal pain, and diarrhea. However, in both situations patients may be infected and asymptomatic.

Laboratory Diagnosis

A presumptive diagnosis of intestinal disease may be based on the patient’s symptoms, particularly with documented ingestion of raw or poorly cooked meat. Confirmation of the diagnosis may depend on finding human fecal specimens containing sporocysts, which are passed in the stool 11 to 18 days after ingestion of beef or pork. Sporocysts of the two Sarcocystis species are very difficult to differentiate.

A muscle biopsy is appropriate for suspected symptomatic intramuscular infection in a patient with a history of travel to or residence in a tropical location. Sarcocysts in biopsy specimens can be identified by microscopy on routine histologic sections stained with hematoxylin and eosin. Most sarcocysts in humans have been found in skeletal and cardiac muscle; however, muscles in the larynx, pharynx, and upper esophagus have also been involved. No molecular assays are currently available for the detection of sarcocystis in humans. However, several amplification methods have been used to detect sarcocystis in intermediate hosts.

Therapy

No known treatment or prophylaxis is available for intestinal infection, myositis, vasculitis, or related lesions caused by human sarcocystosis. Supportive therapy for patients with severe diarrhea is indicated. It is also unclear whether immunosuppressives are effective at reducing the inflammatory reactions seen in vasculitis or myositis. Without more definitive data, no course of therapy currently can be recommended.

Prevention

Cooking meat to an internal temperature higher than 67°C kills Toxoplasma gondii tissue cysts in meat; this temperature should also kill Sarcocystis tissue cysts in meat. Preventing cattle, buffalos, and swine from consuming human feces shedding infective oocysts also prevents animal infection. Most cases of human muscular Sarcocystis infection have been reported from the Far East. When humans are intermediate hosts, preventive measures involve careful disposal of animal feces that may contain the infective sporocysts. This may be impossible in wilder ness areas, where wild animals may serve as reservoir hosts for many Sarcocystis spp.

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